This invention relates to cryogenic refrigerators such as split Stirling cryogenic refrigerators. In particular, it relates to small refrigeration systems having compressors driven by linear motors.
Conventional split Stirling refrigerators usually include a reciprocating compressor and a displacer in a cold finger removed from that compressor. The piston of the compressor is mechanically driven to provide a nearly sinusoidal pressure variation in the pressurized refrigeration gas. The refrigeration gas is usually helium. This pressure variation is transmitted through a supply line to the displacer in the cold finger.
Typically, an electric motor drives the compressor through a crankshaft which is rotatably secured to the compressor. The compressing movement of the compressor causes pressure in the working volume to rise from a minimum pressure to a maximum pressure and, thus, warm the working volume of gas. Heat from the warmed gas is transferred to the environment so that the compression at the warm end of the cold finger is near isothermal. The high pressure creates a pressure differential across the displacer in the cold finger which, when retarding forces are overcome, is free to move within the cold finger. With the movement of the displacer, high pressure working gas at about ambient pressure is forced through a regenerator and into a cold space. The regenerator absorbs heat from the flowing pressurized refrigerant gas and thus reduces the temperature of the gas.
As the compressor piston reverses direction begins to expand the volume of gas in the working space, the high pressure helium in the displacer is cooled even further. It is this cooling in the cold end of the displacer which provides refrigeration for maintaining a time average temperature gradient of over 200 Kelvin over the length of the regenerator.
At some point the decrease in pressure caused by expanding movement of the piston drops sufficiently to overcome retarding forces on the displacer in the cold finger. This causes the displacer to be returned to its starting position. Cold gas from the cold end of the cold finger is driven once again through the regenerator and extracts heat therefrom.
Recently, refrigerators have been proposed and manufactured that depend on linear motors to control the movement of the piston or pistons in the compressor and to control movement of the displacer. The systems use clearance seals between hard ceramic and/or cermet pistons and cylinder liners. An example is disclosed in U.S. Pat. No. 4,545,209, filed by Niels Young on Jan. 17, 1983.
A goal of such linear refrigerators is long life and reduced wear as must be provided for advanced aircraft and spacecraft.